AASHTO Chapter IV b

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AASHTO Chapter IV(b)
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Horizontal Clearance To Obstructions

• Clear zone is the unobstructed flat area provided
  beyond the edge of the traveled way.
• Clear zone widths is related to speed volume,
  and embankment slope.
• For low speed rural collectors and rural local
  roads, a minimum clear zone width of 3.0 m should
  be provided.

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CURBS

• General Considerations
• Curbs serve
• Drainage control
• Roadway edge delineation.
• Right of way reduction
• Aesthetics
• Delineation of pedestrian walkways.
• Reduction of maintenance operations
• Assistance in orderly roadside development.
• Curbs are used extensively on all types of low
  speed urban highways
• Caution should be exercised in the use of crubs
  on high speed rural highways.

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Curb Configurations

• Vertical curbs should not be used along freeways
  or other high speed roadways because an out-of
  control vehicle may overturn or become airborne
  as a result of an impact with such a curb.
• A suitable traffic barrier should be provided
  where redirection of vehicles is needed.
• Extruded curbs provide better initial
  stability,,, easier to construct and are more
  economical that steep faces, Exhibit 4-6C, 4-6E
  and 4-6G.

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Curb Configurations

• Shoulder curbs are placed at the outer edge of
  the shoulder to control drainage, improve
  delineation, control access and reduce erosion.
• For bicyclists using the roadway, sufficient
  width form the face of the curb should be
  provided.
• Gutter may be provided on the traveled-way side
  drainage system, 0.3 to 1.8 m wide, with a cross
  slop of 5 to 8 percent

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Curb placement

• Curbs with low sloping faces may encourage
  drivers to operate relatively close to them.
• While less sloping faces may encourage drivers to
  shy away form them.
• For low speed urban street conditions , curbs may
  be placed at the edge of the traveled way, it is
  preferable that the curbs be offset 0.3 to 0.6 m.

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Curb placement

• Vertical curbs should not be used along freeways
  or other high speed arterials, but if a curb is
  needed, it should be of the sloping type and
  should not be located closer to the traveled way
  than the outer edge of the shoulder.
• When using curbs in conjunction with traffic
  barriers, such as on bridges, the height of a
  vertical curb should be limited to 100 mm or it
  should be of the sloping type.

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Drainage Channels and Sideslopes

• Drainage Channels
• Drainage channels perform the vital function of
  collecting and conveying surface water from the
  highway right-of-way.
• Channels should be protected form erosion. And
  kept clean.
• Channels deterioration can reduce the capacity of
  the channel, which may result in overflow.

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Drainage Channels

• Drainage channels include
• Roadside channels in cut sections to remove water
  form the highway cross section and to control
  surface drainage
• Toe-of-slope channels to convey the water form
  any cut section and form adjacent slopes to the
  natural watercourse.
• Intercepting channels placed back or the top of
  cut slopes to intercept surface water.
• Flumes to carry collected water down steep cut or
  fill slopes.

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Drainage Channels

• The effect of slope combinations and safety is
  important , the severity of traversal of roadside
  channels less than about 1.2 to 2.4 m wide is the
  same for comparable slope combinations regardless
  of channel shape.
• The depth of channel should be sufficient to
  remove surface water without saturation of the
  subgrade.
• The entire roadside channel is visible to the
  driver when the channel sideslope is of 1V4H or
  flatter and a 3.0 m shoulder,

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Drainage Channels

• Storm water should be intercepted and removed as
  rapidly as practical. Sections of channels that
  cross highly permeable soil might need lining
  with impermeable material.
• Median drainage channels are generally shallow
  depressed areas, or swales, located at of near
  the center of the median.
• Flumes are used to carry the water collected by
  intercepting channels down cut slopes and
  discharge the water collected by shoulder curbs.
  Flumes can either be open channels or pipes.
• Channel erosion may be prevented with the use of
  linings that withstand the velocity of storm
  runoff.
• Grass is usually the most economical channel
  lining except on steep slopes.

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Sideslopes

• Sideslopes should be designed to ensure roadway
  stability and t provide a reasonable opportunity
  for recovery for an out-of-control vehicle.
• Three regions of the roadside are important to
  safety
• The top of the slope (hinge point), The hinge
  point contributes to loss of steering control .
• The foreslope region is important where a driver
  could attempt a recovery maneuver or reduce speed
  before impacting the ditch area
• The toe of the slope is often with the roadside
  clear zone and therefore the probability that an
  out of control vehicle will rach the ditch is
  high, a safe transition between fore and
  backslopes should be provided. Exhibit 4-7.

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Sideslopes

• Rounding at the hinge point, can increase the
  general safety of the roadside.
• Foreslopes steeper that 1V4H are not desirable.
• When slopes steeper that 1V3H are used,
  consideration should be given to the use of a
  roadside barrier.
• Flat or rounded natural slopes with good overall
  appearance are appropriate for any roadside
  located near developed and populated areas.

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Sideslopes

• Normally, backslopes should be 1V3H or flatter.
• Retaining walls Should be considered where space
  restrictions would otherwise result in slopes
  steeper than 1V2H.
• On freeway and other arterials with relatively
  wide roadsides, sideslopes should be designed to
  provide a reasonable opportunity for recovery of
  an out of control vehicle.
• rate of slope of 1V6H or flatter on embankments
  can be practical.
• Design slopes for rock vary widely, depending
  upon the materials. A commonly used slope for
  rock cuts is 2V1H , slopes ranging as steep as
  6V1H may be used in good-quality rock.

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ILLUSTRATIVE OUTER CROSS SECTIONS

• Exhibits 4-1 and 4-2 illustrate typical
  combinations of outer cross section elements.

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Traffic Barriers

• General Considerations
• Traffic barriers are used to prevent vehicles
  that leave the traveled way form hitting an
  object that has greater crash severity potential
  that the barrier itself.
• Traffic barriers include both longitudinal
  barriers and crash cushions. The primary function
  of longitudinal barriers is to redirect errant
  vehicles. The primary function of crash cushions
  is to decelerate errant vehicles to a stop.
• Longitudinal barriers are located along the
  roadside and in medians. Bridge parapets or
  rails.
• Longitudinal barriers are generally denoted as
  one of three type Flexible, Semirigid and Rigid
• The major difference between these types is the
  amount of barrier deflection when the barrier is
  struck.

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Traffic Barriers Type of barriers

• Flexible barrier, the cable or beams tear away
  form the support post upon impact. The post
  offers negligible resistance, this system is
  designed primarily to contain rather than
  redirect. Needs more lateral clearance.
• Semirigid system, the posts near the point of
  impact are designed to break or tear away,
  thereby distributing the impact force by beam
  action to adjacent posts.
• A rigid system does not deflect substantially
  upon impact. As the angle of impact increases,
  barrier deceleration forces increase because of
  the absence of barrier deflection. Most
  appropriate where shallow impact angels are
  expected.

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Traffic Barriers

• Important factors
• barrier performance
• lateral deflection
• Space available
• End treatments
• Initial and future maintenance.

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Traffic Barriers

• Six options are available for the treatment of
  roadside obstacles
• Remove or redesign the obstacle.
• Relocate the obstacle to a point where it is less
  likely to be struck.
• Reduce impact by using breakaway device.
• Redirect a vehicle with al longitudinal traffic
  barrier and/or crash cushion.
• Delineate the obstacle.
• Take no action.
• Where curbs and dikes are used to control
  drainage, they should be located flush with the
  face of the barrier or slightly behind it.

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Longitudinal Barriers

• A roadside barrier is a longitudinal system used
  to shield motorists form obstacles or slopes
  located along either side of a roadway.
• The height and slope of an embankment are the key
  factors in determining barrier.
• Barriers should be located beyond the edge of the
  shoulder to ensure that the full shoulder width
  may be used. The fill sporting the barrier should
  provide lateral support .
• To provide safe barriers, ends may be buried,
  covered with a mound or earth, flared back, or
  protected with a crash cushion or an approved
  crash tested terminal.

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Median Barriers

• Common types of median barrier include
  double-faced steel W-beam (blocked-out) installed
  on strong posts, box beam installed on weak
  posts, and concrete barrier.
• Less common type of median barrier include two-or
  three-cable barrier installed on light steel
  posts, double-faced steel W-beam installed on
  weak posts, double-faced steel three-beam
  (blocked-out) installed on strong posts and
  cable-chainlink fence combination.

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Median Barriers

• In selecting the type of Median Barriers, the
  maximum deflection should be less than one-half
  the median width to prevent penetration into the
  opposing lanes of traffics. The Median Barriers
  should be designed to redirect the vehicle.
• On heavily traveled facilities, a concrete
  barrier with a sloping face deflects a vehicle
  striking it at a slight impact angel and needs
  little maintenance.

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Median Barriers

• The appropriate types of median barriers are
  different for stepped median. Cable, W-beam on
  weak posts, and box-beam systems are generally
  limited to relatively flat medians and may not be
  appropriate for some stepped median sections.
• Precast concrete median barrier can be used for
  temporary protection of work areas, guiding
  traffic during constriction and can be
  incorporated permanently as part of the completed
  facility.